1. Field of the Invention
The present invention relates to a printed circuit board supporting structure, and more particularly to a printed circuit board supporting structure in which a solder bump pattern provided on a board surface of a printed circuit board is brought into contact with an obverse surface of a board support constituted by a conductor such as a chassis and is made electrically conductive therewith, and in which the printed circuit board is fixed to the board support at a portion of contact between the solder bump pattern and the board support.
2. Description of the Related Art
Conventionally, a board fixing device has been known for facilitating the removal of a board from a chassis when an apparatus is disassembled (e.g., refer to JP-A-2002-299860). In addition, as a method of fixing a printed board on a mounting board, a method has been known in which the printed board is clamped by a clamping piece and a base portion of a mounting device mounted on the mounting board by the use of a tightening force of a screw, or a method in which the printed board is screwed down to a mounting piece formed in a mounting board by being cut out (e.g., refer to JP-UM-A-1-160882).
Meanwhile, a structure for connecting a circuit board to chassis ground has been known in which, when the circuit board is fastened and fixed to the chassis ground by using a fixing screw, solder formed on the circuit board is brought into pressure contact with the chassis ground at a fastening and fixing portion (e.g., refer to JP-A-2000-216565). In addition, a structure has also been known in which a printed circuit board provided with grounding solder around a mounting hole is screwed down to a boss provided on a chassis (e.g., refer to JP-A-2002-217502).
In a case where a printed circuit board supporting structure is adopted in which the printed circuit board is supported by and fixed to a board support such as a metal chassis at a plurality of portions, and the printed circuit board and the metal chassis are electrically connected at these fixing portions, there have been cases where the structure shown in FIGS. 6 and 7 is adopted. In the printed circuit board supporting structure shown in the drawings, a screw insertion hole 12 and a solder bump pattern 2 formed around its periphery on a board surface are provided on each corner portion of a printed circuit board 1. Meanwhile, a board support 4 is provided on a metal chassis 3, and a headed clamp screw 5 inserted in the screw insertion hole 12 of the printed circuit board 1 placed on the board support 4 is screwed and tightened in a threaded hole 41 provided in the board support 4. By making use of the tightening force of the headed clamp screw 5 of such a measure that the respective solder 21 of the solder bump pattern 2 is crushed, the solder bump pattern 2 and the obverse surface of the board support 4 are brought into pressure contact with each other while a gap S is being secured between the printed circuit board 1 and the board support 4, thereby allowing the printed circuit board 1 and the board support 4 to be electrically conductive.
According to this printed circuit board supporting structure, there are advantages in that the screwing down of the printed circuit board 1 to the board support 4 and the making of the printed circuit board 1 electrically conductive with the board support 4 are realized simultaneously, and that reliability in electrical continuity and stability in electrical continuity improve.
However, if the printed circuit board supporting structure explained with reference to FIGS. 6 and 7 is applied to a plurality of portions of the printed circuit board 1, e.g., the respective ones of the four corner portions of the rectangular printed circuit board 1, there are cases where variations occur in the gap S between the printed circuit board 1 and the board support 4 owing to such as variations in the tightening torque based on the headed clamp screw 5 and variations in the height of the bump of the solder 21 of the solder bump pattern 2. If the variations occur in the gap S, even if the height of the board support 4 corresponding to the four corner portions of the printed circuit board 1 is in agreement with high accuracy, the dimensional accuracy in the mounting of the printed circuit board 1 declines, and warpage occurs in the printed circuit board 1, so that a bending stress is likely to remain.
Meanwhile, as the printed circuit board 1, one having a ball grid array (BGA) 6 mounted thereon, as shown in FIG. 8, is known. The BGA 6 has terminals and solder balls arrayed on its lower surface, and when it is mounted on the printed circuit board 1, the terminals and the conductor pattern of the printed circuit board 1 are connected by melting the solder balls. Therefore, if the printed circuit board 1 with the BGA 6 mounted thereon has been irregularly warped or deformed, there occurs the possibility that the BGA 6 floats up from the board surface of the printed circuit board 1 over time, resulting in the exfoliation or breakage of portions of connection between the terminals and the conductor pattern. This constitutes a factor causing the reliability of the apparatus to decline substantially. It should be noted that, in FIG. 8, reference character A denotes a mounting portion using the supporting structure explained with reference to FIG. 6, and reference numeral 12 denotes the screw insertion hole 12, and numeral 2 denotes the solder bump pattern.
In this respect, JP-A-2002-299860 and JP-UM-A-1-160882 merely provide structures for fixing the printed circuit board to the chassis or the like, while JP-A-2000-216565 and JP-A-2002-217502 merely cause the solder bump portions of the printed circuit board to electrically conduct with the board support by adopting structures similar to those explained with reference to FIG. 6 or 7. For these reasons, even if the techniques disclosed in these patent documents are combined in any way, it is impossible to prevent the warpage of the printed circuit board which can possibly occur in the case where the printed circuit board and the metal chassis are electrically connected at fixing portions of the printed circuit board with respect to the board support.
Accordingly, in the printed circuit board supporting structure explained with reference to FIGS. 6 and 7, it is conceivable to adopt a supporting structure in which the aforementioned gap S is prevented from occurring by directly superposing the lands on the board surface of the printed circuit board 1 on the board support 4 by omitting the solder bump patterns 2. If this structure is adopted, since the gap does not exist, there is an advantage in that the dimensional accuracy in the mounting of the printed circuit board 1 improves, making it difficult for the warpage to occur in the printed circuit board 1. On the contrary, however, there arises a disadvantage in that reliability in the electrical continuity and stability in electrical continuity between the printed circuit board 1 and the board support 4 decline. To compensate for this disadvantage, it has been practiced to establish electrical continuity with the board support 4 by means of separate parts such as lead wires from the printed circuit board 1. In such a case, however, there arise drawbacks in that the lead wires are additionally needed, and the number of parts and the number of steps in operation increase, resulting in higher manufacturing cost, and that since the electrical connection path becomes long, the connection impedance becomes high, resulting in a decline in electrical performance.
In addition, it has also been practiced to electrically connect a conductor pattern on the obverse surface of the printed circuit board having conductor patterns on both obverse and reverse surfaces thereof to a chassis by means of a metal screw. However, this also results in a relatively long electrical connection path, and there is a possibility of exerting an adverse effect on the connection impedance in a high-frequency region, in particular.
Accordingly, an object of the invention is to provide a printed circuit board supporting structure which makes it possible to eliminate the possibility of occurrence of warpage in the printed circuit board by basically adopting a supporting structure such as the one explained with reference to FIG. 6 or 7, which has the advantages that there is no need to use additional parts such as lead wires, an increase does not occur in the connection impedance, and no adverse effect is exerted on the connection impedance in the high-frequency region, i.e., a printed circuit board supporting structure in which the printed circuit board is clamped and fixed to the board support constituted by a conductor by means of a clamping device, and the solder bump pattern provided on the board surface of the printed circuit board is thereby brought into pressure contact with the obverse surface of the board support at that clamped portion.